There is a general desire to increase the energy efficiency of internal combustion engines used for powering of vehicles. One way of doing this is to utilize the waste heat of the engine using a Rankine system where a recirculating fluid in an evaporation step takes up heat from the exhaust gas or the cooling water of the engine and returns the energy/power to the engine via e.g. an expander turbine connected to a generator. A system of this type also includes a condenser and a pump for recirculating the fluid. An example of such a system is disclosed in US2015/0176482.
Systems of the above type typically include a by-pass passage for allowing the fluid to fully or partly by-pass the expander during recirculation. Without such a by-pass passage, evaporated fluid may push liquid fluid forward in the system towards the expander that may be damaged if designed to handle only fluid in vapour form. A by-pass valve is used to switch between (partial) by-pass flow and operation flow where the fluid passes through the expander. When building up of the pressure in the system during a starting phase the by-pass valve is typically set in a partially open position to provide a certain back pressure and a partial flow through both passages. When the pressure and temperature have reached a certain level the valve is closed and the system can start operating.